The document you get for a CC is not really a "doctor signed medical document", but rather a "document signed CC document". It was made by the CCs and is just used to cover their ass when the police come looking to make sure they're not selling recreationally or in any other capacity to non-patients. Same reason they keep your photo ID on file - it's to save their ass so they can continue to offer a great service to other patients.
You've never really understood what is and isn't legal though, so I doubt I'll be able to start educating you now.